Hybrid circuit assemblies are known in the art and comprise several separate component parts which are mounted or attached to a substrate. The components are interconnected either by wire bonds, direct solder attachment or electrically conductive epoxy. Once assembled, the circuit is covered with an encapsulant, for example, a known material such as liquid epoxy. The primary purpose of the encapsulant is to isolate and protect the components and bonds from moisture and other contaminants and mechanical shock and corrosion.
The principal drawback of conventional encapsulation processes is the creation of complex stress distributions within the assembly. The stress distributions are a result of the differential shrinkage between the encapsulant and the substrate. These stresses can warp the substrate and result in the eventual delamination of the plastic encapsulant from the substrate surface thereby exposing the hybrid circuit to external contaminants.
Warpage also gives rise to problems associated with the manufacture of hybrid circuits. Warping can reduce the manufacturing yield of circuits. This problem is especially prevalent for larger hybrid circuits where the warpage can affect the ability to mount the circuit on a planar surface, for example, a printed circuit board, therefore necessitating rejection of the warped hybrid assembly. For smaller size hybrid circuits, the effects of warpage are usually not so serious and mainly impact the efficiency of the manufacturing process. Post cure processing also generally requires a relatively planar circuit assembly and thus warpage can affect the manufacturing efficiency for both small and large hybrid circuit assemblies. This problem is further exacerbated by the fact that the resulting warpage is directly proportional to the surface area of the hybrid circuit which is covered by the encapsulant material. Therefore one method of reducing warpage in conventional hybrid circuits involves reducing the area which is encapsulated. While this method can alleviate the effects of warpage, it results in higher unit costs due to reduced surface area utilization and density. It does not solve the root problem.
Other attempts have been made to minimize the stresses which cause warpage. The emphasis in the art has been on the development of "low stress" encapsulant materials. Encapsulant materials, such as the HYSOL.TM. FP4400 series of encapsulants, have been produced which exhibit shrinkage and thermal coefficients of expansion which are closer to the properties of the material used for the substrate. The aim of such attempts is to reduce warpage by eliminating the resultant differential stresses between the encapsulant and the substrate.